Large shaft driven machines, such as gas turbine engines used in aircraft applications, require radial bearings and thrust bearings to support and position the shafts thereof. A typical gas turbine engine may have two or three coaxial shafts nested within each other, resulting in a shaft support design which only permits the inner shaft to be supported at the ends thereof. Deflection of the shaft between the support bearings is inevitable due to the mass of the compressor and turbine rotors. If a shaft deflects significantly, or "bows" (i.e. the displacement of the shaft from the desired axis of rotation varies with the axial position along the shaft) wear or damage may occur to the engine components. This deflection is referred to in the art as the "first bending critical".
If the shaft of a gas turbine engine bows beyond allowable operating tolerances, the tips of the rotating blades may grind into the outer air seal, thereby wearing away both the seal and the tips of the blades and deteriorating overall operating efficiency of the engine. For land-based applications where weight is not a critical factor, the wall of the inner shaft can be increased as desired to ensure that the deflection is within tolerances at all anticipated operating conditions. However, for high performance aircraft, which are subjected to varying "G-forces" during flight maneuvers, it is imperative that the deflection be controlled while minimizing any increase in the weight of the shaft.
Magnetic bearings are an attractive option to the ball or roller bearings which are commonly used on aircraft gas turbine engines, due to the potentially frictionless support they provide. However, current magnetic bearings do not solve the shaft deflection problems discussed above.
What is needed is a magnetic bearing assembly which reduces the deflection, and hence the first bending critical, of a rotating shaft without significantly increasing the weight of the gas turbine engine.